In bottom up nanotechnology molecules must be “seen”, “felt”, and manipulated. These issues have been central in the research of MTP using Atomic Force Microscopy platforms. In related projects, a significant effort is underway to study single molecule interaction forces in supramolecular systems, including H-bonded complexes. A central issue regards tuning the lifetime of complexes. For this purpose 2-ureido-4[1H]-pyrimidinone moieties at gold (Au) surfaces were studied (Figure 5).
In a collaborative project carried out together with the group of Prof. Wolfgang Knoll at the MPI-P Mainz, Germany, we have shown that surfaces modified with novel asymmetric 2-ureido-4[1H]-pyrimidinone-hydroxyalkane disulfide adsorbates exhibit efficient and controllable self-complementary molecular recognition of the pyrimidinone moieties. Depending on the choice of solvent and temperature, reversible supramolecular recognition between surface-immobilized adsorbates and external guests occurred, as shown by several methods including atomic force microscopy (AFM). The convenient control of the complex stability renders these systems highly suitable for the construction of more complex, directionally defined surface-immobilized supramolecular architectures, as well as for the study of pyrimidinone-based supramolecular polymers at surfaces using AFM approaches. This system also allows us to investigate single-molecule rupture forces in thermodynamic equilibrium, as well as in non-equilibrium states. An example for single-molecule polymer stretching (the polymeric PEG spacer that decouples specific and non-specific interactions) and rupture of the quadruple H-bond is shown in Figure 6.

Figure 5. The complex formation of 2-ureido-4[1H]-pyrimidinone moieties at Au surfaces through quadruple hydrogen bonds can be conveniently controlled via solvent or temperature.